1. Technical Field
The present invention relates to a reception apparatus, a transmission apparatus, a reception method and a transmission method.
2. Description of the Related Art
In recent years, accompanying the adoption of multimedia information in cellular mobile communication systems, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data. Furthermore, studies have been actively conducted in LTE-Advanced (Long Term Evolution Advanced) to realize high transmission rates by utilizing broad radio bands, Multiple-Input Multiple-Output (MIMO) transmission technology, and interference control technology.
In addition, taking into consideration the introduction of various devices as radio communication terminals in M2M (machine to machine) communication and the like as well as an increase in the number of multiplexing target terminals due to MIMO transmission technology, there is a concern regarding a shortage of resources in a mapping region for PDCCH (Physical Downlink Control Channel) that is used for a control signal (that is, a “PDCCH region”). If a control signal (PDCCH) cannot be mapped due to such a resource shortage, data cannot be assigned to the terminals. Therefore, even if a resource region in which data is to be mapped is available, the resource region may not be used, which causes a decrease in the system throughput.
As a method for solving such a resource shortage, a study is being carried out on assigning, also in a data region (that is, “PDSCH (Physical Downlink Shared CHannel” region), control signals for radio communication terminal apparatuses (hereunder, abbreviated as “terminals,” UE (User Equipment)) served by a radio communication base station apparatus (hereunder, abbreviated as “base station”). A resource region in which control signals for terminals served by the base station are mapped is referred to as an Enhanced PDCCH (ePDCCH) region, a New-PDCCH (N-PDCCH) region, an X-PDCCH region or the like. Mapping the control signal (i.e., ePDCCH) in a data region as described above enables transmission power control on control signals transmitted to a terminal near a cell edge or interference control for interference by a control signal to another cell or interference from another cell to the cell provided by the base station.
Further, according to the LTE-Advanced system, in order to expand the coverage area of each base station, relay technology has been studied in which a radio communication relay station apparatus (hereunder, abbreviated as “relay station”) is installed between a base station and terminals, and communication between the base station and terminals is performed via the relay station. The use of relay technology allows a terminal that cannot communicate with the base station directly to communicate with the base station via the relay station. According to the relay technology that has been introduced in the LTE-Advanced system, control signals for relay are assigned in a data region. Since it is expected that the control signals for relay may be extended for use as control signals for terminals, a resource region in which control signals for relay are mapped is also referred to as an “R-PDCCH.”
In the LTE (Long Term Evolution) system, a DL grant (also referred to as “DL assignment”), which indicates a downlink (DL) data assignment, and a UL grant, which indicates an uplink (UL) data assignment are transmitted through a PDCCH.
In LTE-Advanced, a DL grant and a UL grant are mapped to R-PDCCH as well as PDCCH. In the R-PDCCH, the DL grant is mapped in the first slot and the UL grant is mapped in the second slot (refer to NPL 1). Thus, each relay station monitors (blind-decodes) control signals transmitted using an R-PDCCH from a base station within a resource region indicated by higher layer signaling from the base station (i.e., a “search space”) and thereby finds the control signal intended for the corresponding relay station.
In this case, the base station indicates the search space corresponding to the R-PDCCH to the relay station by higher layer signaling as described above.
In the LTE and LTE-Advanced systems, one RB (resource block) has 12 subcarriers in the frequency domain and has a width of 0.5 msec in the time domain. A unit in which two RBs are combined in the time domain is referred to as an RB pair (for example, see FIG. 1). That is, an RB pair has 12 subcarriers in the frequency domain, and has a width of 1 msec in the time domain. When an RB pair represents a group of 12 subcarriers on the frequency axis, the RB pair may be referred to as simply “RB.” In addition, in a physical layer, an RB pair is also referred to as a PRB pair (physical RB pair). A resource element (RE) is a unit defined by a single subcarrier and a single OFDM symbol (see FIG. 1).
PDCCH and R-PDCCH have four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, see NPL 1). Levels 1, 2, 4, and 8 have, for example, six, six, two, and two “mapping candidates,” respectively. As used herein, the term “mapping candidate” refers to a candidate region in which a control signal is to be mapped, and a search space is formed by a plurality of mapping candidates. When a single aggregation level is configured for a single terminal, a control signal is actually mapped in one of the plurality of mapping candidates of the aggregation level. FIG. 2 illustrates an example of search spaces corresponding to an R-PDCCH. The ovals represent search spaces for the aggregation levels. The multiple mapping candidates in each search space for each aggregation level are located in a consecutive manner on VRBs (virtual resource blocks). The resource region candidates in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
Studies are being conducted with respect to individually configuring search spaces corresponding to the ePDCCHs for terminals. Further, with respect to the design of the ePDCCHs, part of the design of the R-PDCCH described above can be used, and a design that is completely different from the R-PDCCH design can also be adopted. In fact, studies are also being conducted with regard to making the design of the ePDCCHs and the design of R-PDCCHs different from each other. In the following description, mapping candidates in a search space corresponding to ePDCCH may be called “ePDCCH candidates.”
As described above, a DL grant is mapped to the first slot and a UL grant is mapped to the second slot in an R-PDCCH region. That is, a resource to which the DL grant is mapped and a resource to which the UL grant is mapped are divided on the time axis. In contrast, for the ePDCCHs, studies are being conducted with regard to dividing resources to which DL grants are mapped and UL grants are mapped on the frequency axis (that is, subcarriers or PRB pairs), and with regard to dividing REs within an RB pair into a plurality of groups.
In addition, “localized allocation” which allocates ePDCCHs collectively at positions close to each other on the frequency band, and “distributed allocation” which allocates the ePDCCHs by distributing ePDCCHs on the frequency band have been studied as allocation methods for ePDCCHs (for example, see FIG. 3). The localized allocation is an allocation method for obtaining a frequency scheduling gain, and can be used to allocate an ePDCCH to a resource that has favorable channel quality based on channel quality information. The distributed allocation distributes ePDCCHs on the frequency axis, and can obtain a frequency diversity gain. In the LTE-Advanced system, both a search space for localized allocation and a search space for distributed allocation may be configured (for example, see FIG. 3).
LTE-Advanced defines transmission methods such as transmission through single antenna port precoding and transmission through precoding using multiple antenna ports (e.g., see NPLs 2 and 3)
In the following description, transmission through single antenna port precoding may be called “single antenna port transmission (“One Tx port”)” and transmission through precoding using multiple antenna ports may be called “transmission diversity using multiple antenna ports (“Multi ports Tx diversity” or simply “Tx diversity”).” In the following description, the term “precoding” refers to assigning a weight to a transmission signal (multiplying a transmission signal by a weight) per antenna port or antenna. In addition, the term “layer” refers to s each of spatially multiplexed signals and may also be called “stream.” Moreover, the term “rank” represents the number of layers. Furthermore, the term “transmission diversity” generically refers to transmission of data using a plurality of channels or a plurality of resources. By applying transmission diversity, signals are transmitted through channels (resources) including good channels (resources) and poor channels (resources), and it is thereby possible to obtain average receiving quality. That is, the transmission diversity makes receiving quality stable without causing it to degrade considerably. For example, channels or resources used in transmission diversity are frequency, time, space, antenna ports and beams.
[Single Antenna Port Transmission]
In single antenna port transmission, a base station selects precoding based on feedback information indicating channel quality measured by a terminal (also referred to as “closed-loop precoding” or “feedback-based precoding”). For this reason, single antenna port transmission is a transmission method which is effective when feedback information highly reliable, for example, when the moving speed of a terminal is relatively low. However, when feedback information cannot be obtained or when the terminal move relatively fast so that the feedback information is not very reliable, the base station may select optional precoding (open-loop processing).
For example, single antenna port transmission is applicable to antenna port 1 (CRS (Cell specific Reference Signal)), antenna port 4 (MBMS (Multimedia Broadcast Multicast Service)), antenna port 5 (UE specific RS), antenna port 7 (DMRS (Demodulation Reference Signal)) and antenna port 8 (DMRS).
[Transmission Diversity Using Multiple Antenna Ports]
Transmission diversity using multiple antenna ports can obtain a diversity gain without requiring feedback information. For this reason, transmission diversity using multiple antenna ports is a transmission method which is effective when the terminal moves relatively fast so that the channel quality varies drastically, or when channel quality is poor so that a diversity gain is necessary.
An example of transmission diversity using multiple antenna ports used at rank 2 or higher is large delay CDD (Cyclic Delay Diversity) (spatial multiplex+transmission diversity). On the other hand, transmission diversity using multiple antenna ports used at rank 1 is, for example, spatial frequency block coding for 2 antenna ports (SFBC: Space Frequency Block Code) and SFBC-FSTD (Frequency Switched Transmit Diversity) for 4 antenna ports.
For example, transmission diversity using multiple antenna ports is applied to antenna ports 1 and 2 (CRS) and antenna ports 1, 2, 3 and 4 (CRS). Note that transmission diversity using multiple antenna ports is supported in CRS, but not supported in DMRS.